That's due to different factors. One is that according to Hodgdon, IMR changed processes about 2010 or so and will no longer make the IMR powders by the original process. There was also a formulation problem, the European Union having banned some of the traditional chemistry as carcinogenic, the dibutyl phthalate deterrent, in particular, so they may have changed the chemistry for compatibility with NATO. No details were given, and the proprietary nature of some processes means they may never be available if they weren't specified by the military. The bottom line is that what we get now is as close to the original IMR powders as the new process and formulation practices can produce.
Another factor is that a lot of old load data was developed in production guns by commercial bullet makers by watching for pressure signs, and a lot of that old data has turned out to produce pressures exceeding SAAMI's values when fired in the minimum chamber pressure test guns. Pressure-tested Lyman data was an exception, but Speer and Hornady and others had some pretty hot data in the old books. If you look at recent photos of Hornady's plant, you can now see what appear to be strain gauges mounted on their test guns. Strain gauges produce results at least as repeatable as the piezo transducers used in the SAAMI standard but are a lot less expensive. Nobody trusts the old pressure signs anymore.
Still another thing that has changed is that while the SAAMI standard only requires pressure peak reading instrumentation, in this day and age of high speed data acquisition, most industry people who use test barrels have software that produces a full trace of the pressure curve on a computer screen. That is not something that could be done with crushers. I've heard this practice has revealed some anomalies with certain combinations of components causing some to be dropped.
Another factor is that a lot of old load data was developed in production guns by commercial bullet makers by watching for pressure signs, and a lot of that old data has turned out to produce pressures exceeding SAAMI's values when fired in the minimum chamber pressure test guns. Pressure-tested Lyman data was an exception, but Speer and Hornady and others had some pretty hot data in the old books. If you look at recent photos of Hornady's plant, you can now see what appear to be strain gauges mounted on their test guns. Strain gauges produce results at least as repeatable as the piezo transducers used in the SAAMI standard but are a lot less expensive. Nobody trusts the old pressure signs anymore.
Still another thing that has changed is that while the SAAMI standard only requires pressure peak reading instrumentation, in this day and age of high speed data acquisition, most industry people who use test barrels have software that produces a full trace of the pressure curve on a computer screen. That is not something that could be done with crushers. I've heard this practice has revealed some anomalies with certain combinations of components causing some to be dropped.
